Pancreatic islet transplantation remains an attractive potential form of therapy for patients with diabetes mellitus. However, allografts of donor human islets have not been successful (Hering, B. et al. Horm Metab Res 1988 20:537). Availability and yield of viable, isolated human islets is likely to remain extremely limited. Ultimately, islet graft therapy for large numbers of patients will require the use of donor islets harvested from animals (xenografts). The only animals that are relatively closely related to man (that is, concordant xenografts) are the subhuman primates. Unfortunately, availability of these potential donors of islets is extremely limited. Therefore, it is logical to investigate the potential utility of a variety of widely unrelated animal sources of islets (discordant xenografts). Microencapsulation of donor islets is the most promising approach to long-term survival of islet xenografts.